(PUP-3236) Support PUPPET_LOADED env var in Gemfile

This commit adds support for an environment variable, PUPPET_LOADED,
to be set during bundle installation.  If set, the variable will
prevent the puppet gem from being loaded; the use case is for test
environments where you already have puppet available on the load path.
(PUP-1680) Set accept-encoding to identity when http_compression is false

Override the Ruby 2.x default of setting accept-encoding to gzip when puppet http_compression is set to false.
correct test description for http_compression disabled
(MAINT) Perform more thorough cleanup on master.

Signed-off-by: Wayne <wayne@puppetlabs.com>